1. Field of the Invention
The present invention relates to a method of replacing a rotary cylinder on which magnetic rotary heads are mounted, in a rotary cylinder device of a video tape recorder (hereinafter called VTR) or the like.
2. Description of the Prior Art
Recently there is a mounting demand for recording and reproducing apparatus capable of recording information with high density and transmitting information at high rates. Such apparatus include digital VTRs and VTRs for high definition television (for example, Hi-Vision) in which the picture quality is very much enhanced in comparison with existing television systems. In such apparatus in which information on a magnetic tape is recorded and reproduced by magnetic rotary heads, the quantity of information which can be recorded in an area of magnetic tape is referred to as the recording density. Narrowing the tracks recorded on a magnetic tape has been considered in connection with increasing recording density. Moreover, the width of the frequency band and the wavelengths of the signals to be recorded will depend upon the width of the recording tracks. Therefore, it is necessary to increase the rotating speed of the rotary cylinder of the apparatus the larger the width of the frequency band and the smaller the wavelengths of the recorded signals become. At higher density recording, however, the magnetic rotary head may fail to trace the track correctly at the time of reproduction of the signals, or the noise produced by the bearing may increase with increases in the rotary speed of the rotary cylinder. Accordingly, a hydrodynamic bearing that is more precise and less noisy than a ball bearing is used in the rotary cylinder device.
The apparatus also includes a stationary cylinder for guiding a magnetic tape, and a stationary shaft having herringbone grooves at two positions on the outer circumference thereof and at an upper end surface thereof. The stationary shaft is press-fitted to a central part of the stationary cylinder. A sleeve mounted coaxially with and rotatable relative to the stationary shaft has a bearing surface confronting the herringbone grooves, and is supported by a disk. A thrust plate has a spiral groove in its lower end surface, and is fastened with a screw to the upper end surface of the sleeve so as to confront the upper end surface of the stationary shaft. A small gap between the herringbone groove and the bearing surface, and a small gap between the upper end surface of the stationary shaft and the spiral groove are filled with lubricant. The rotary cylinder carries magnetic rotary heads which project a specified amount so as to contact the magnetic tape. This cylinder extends coaxially with the disk, and is detachably fixed thereto with a screw. A rotary transformer for transmitting a signal includes a stationary portion and a rotary portion. A motor for rotating and driving the rotary portion comprises a motor magnet fixed to the disk, and a motor coil fixed to the stationary cylinder.
A method of detaching and attaching the rotary cylinder will be described below.
When an electric current flows in the motor coil, and the rotary portion of the device (including the sleeve) rotates, the centripetal nature of the rotary portion is maintained by the pumping action of the radial bearing composed of the herringbone groove, bearing surface, and lubricant. Furthermore, the thrust bearing, composed of the spiral groove, upper end surface of the stationary shaft, and lubricant, performs a pumping action that ensures that the rotary portion floats at a stable position on the stationary portion of the device, whereby normal rotation is maintained. However, the magnetic rotary heads become worn over time due to contact of the magnetic tape and the magnetic rotary heads, whereupon the extent to which the heads project from the rotary cylinder becomes smaller such that the quality of the recording and reproducing becomes unsatisfactory. Therefore, when the magnetic rotary heads are worn, generally, the rotary cylinder on which the magnetic rotary heads are mounted is replaced with a new one. In such a case, to dismount the rotary cylinder, the screw is loosened from the disk, and the rotary cylinder is pulled from the disk upward while pressing downward on the upper end surface of the thrust plate. On the contrary, to mount the new rotary cylinder, the inner circumference of the rotary cylinder and the outer circumference of the disk are coaxially fitted to another, and a screw is inserted into the threaded hole of the disk.
In such a method, however, dynamic pressure is not present in the bearing because the rotary portion has been stopped. Therefore, the outer circumference of the shaft and the bearing surface of the sleeve, and the lower end surface of the thrust plate and the upper end surface of the shaft contact each other. Accordingly, when detaching and attaching the rotary cylinder and disk, the sleeve, thrust plate and shaft nearly contact each other, and an external force is transmitted by the screw to the bearings. As a result, the sleeve or thrust plate may be damaged, and the rotary portion may not rotate normally after replacement, and the bearing life is also shortened. Furthermore, damage to the radial bearing causes the shaft of the stationary portion to become loose, and therefore the tracking of the magnetic tape by the magnetic rotary heads is impaired.